A mobile communication system (hereinafter, a MIMO system) using a multiple input multiple output (MIMO) scheme transmits data through individual channels that are formed among multiple antennas at transmitting/receiving terminals to thus acquire greater data rates than a system using a single antenna.
Assuming that the transmitting terminal or the receiving terminal knows channel information in the MIMO system, interference signals can be removed through an appropriate detection scheme or a precoding scheme.
The MIMO system is applicable to a cellular system simultaneously supporting multiple users. Since it is generally impossible for user terminals to communicate cooperatively, a base station transmits signals through a precoding method so as to prevent interference between the user terminals.
The method for removing inter-user interference in the multiple-user supporting MIMO system is originated from the dirty paper coding method. According to the dirty paper coding method, the inter-user interference can be completely eliminated without any loss of transmittable data, but since the method is excessively complicated, a simplified nonlinear method such as the Tomlinson-Harashima precoder (THP) or a linear method such as block diagonalization is used.
The block diagonalization method shows relatively simple and good performance against errors, but increases loss of channel capacity when the number of users who are simultaneously supported is increased. On the contrary, the THP method has the advantage of less loss of channel capacity even when the number of users is increased, but the system has considerable complexity and its performance against errors is worsened when there are relatively lesser number of users in the system.
Among the methods for improving the above-noted drawbacks of the block diagonalization method and the THP method, there are the block THP (BTHP) method for eliminating inter-user interference so as to improve error processing performance of the THP by using the user's multiple antennas, and the maximum likelihood (ML) receiving method designed for the same.
The ML-applied BTHP method generates excellent error processing performance compared to the existing THP methods, but provides degraded error processing performance compared to the ML-applied block diagonalization method, and each user terminal must cover the great complexity of the ML receiving method.
Therefore, a method for reducing the complexity of the ML-applied BTHP in the user terminal or moving the complexity to the base station which is relatively robust for it while increasing error processing performance of the ML-applied BTHP is required.
The vector perturbation (VP) method, similar to the THP method, infinitely increases an equivalent transmission symbol through a modulo operation, and transmits a symbol having the least power when the channel's inverse matrix is multiplied thereto.
The VP method is a kind of nonlinear channel inversion (CI) method and generates the same effect of moving the ML receiving method to the base station, and the user terminal can finish detection through the modulo operation without channel information. However, since the method steeply increases the complexity as the number of the antennas is increased in a like manner of the ML method, it is difficult to apply the method to the multi-user MIMO system.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.